The invention relates to a multilayer adhesive tape having a backing based on an oriented thermoplastic film coated on one side with adhesive composition. The adhesive tape indicates the unauthorized opening of a package by virtue of the fact that, on peeling from the surface to which it is stuck, areas of the adhesive tape are extracted from the adhesive tape and remain on the surface as a message, which is imperceptible prior to detachment from the surface. The message remains visible even if following detachment the adhesive tape is replaced exactly at the original bond site.
Protecting goods present in the logistical chain from producer to user has become more difficult owing to the large number of links in this chain. Thus the damage which occurs by theft of the product from its delivery packaging during warehousing and transit has risen to become a significant cost factor. The recipient of a delivery unit in which some of the product has been switched or removed by theft, moreover, has difficulties in proving that this act has not been committed within his or her own sphere of influence.
Significantly greater security against an attempted theft, and reliable evidence that a delivery unit has been opened before its transfer to the next section of the logistics chain, are made possible by an adhesive tape which indicates by means of an irreversible message that it has already been detached from the surface of the delivery unit.
Security systems for detecting unauthorized opening are known and are described in multiplicity in the prior art. Security labels, in particular, are of great importance in this context. For example, U.S. Pat. No. 4,184,701 describes security labels which are constructed from a plurality of color layers or, generally, layers and which as a result exhibit little adhesion of the product layers to one another. In combination with a strongly adhering adhesive composition, such layers separate from one another irreversibly, thereby indicating the broaching of the package.
Furthermore, for example, DE 34 31 239 A1 describes the use of security labels comprising a backing material of low tear propagation resistance. In combination with a strongly adhering adhesive composition, the security label is irreversibly destroyed on removal and thus indicates the attempted broaching.
The security label has a lower, first adhesive layer, intended for detachment from a backing and application to the package, on the bottom face of a first label material to whose top face there is applied, by means of a second adhesive layer, a second label material. The extent of the bond strength of the first adhesive layer between the package and the first label material is different than the extent of the bond strength of the second adhesive layer between the first label material and the second label material.
Adhesive security tapes are commonly designed in such a way that a weakness in anchoring below the colored structure of the adhesive tape is introduced in the region of the writing. For instance, U.S. Pat. No. 5,633,058 describes a transparent printed indicium, anchored weakly to the backing film, which is covered with a full-area colored layer which anchors well to the backing film and to the first print. The colored layer is covered with a self-adhesive composition. When an adhesive security tape of this kind is peeled from a surface, the transparent printed indicium is parted from the film surface, together with the color layer and the adhesive composition, and remains as writing on the surface.
EP 0 491 099 A1 describes a printed indicium comprising a silicone material likewise covered with a colored layer which adheres well to the film. The colored layer is covered with a self-adhesive composition. When an adhesive security tape of this kind is peeled from a surface, the color layer and the adhesive composition detach from the silicone surface and remain as writing on the surface.
The disadvantage of a construction of this kind is the complex process engineering for producing the layers, since partial silicone surfaces greatly hinder further coating, owing to the low surface tension.
U.S. Pat. No. 4,121,003 describes how the transfer of writing is brought about by this writing comprising of a material of low cohesion which, when the adhesive security tape is detached, splits within itself and remains in part on the surface to which it was stuck.
EP 0 404 402 A2 is based on a transparent splittable layer which becomes opaque at the boundary layer which develops, and which therefore offers a previously imperceptible optical contrast surface against which the writing then becomes legible.
A disadvantage of structures of this kind, moreover, is that the surface of delivery units which comprise not very split-resistant material such as paper, for example, are easily torn by the adhesive tape. The split-off surface of the delivery unit tears over its full area beneath the adhesive tape and adheres in such a way that it covers the surface of the adhesive composition. In that case, the writing is not transferred to the surface of the cardboard box, and the message provided does not appear.
This disadvantageous circumstance is addressed by many available adhesive tapes of this kind by virtue of the fact that they possess a weakly adhering adhesive composition which may be peeled off without tearing the paper surface. The disadvantage of this solution, however, is inadequate bonding reliability to the different kinds of paper materials, especially grades with a high waste paper content, so that the U-shaped closure of a delivery case carrying these adhesive tapes is parted again under the pressure of the lid flaps, and the case opens of itself.
A further requirement of security against manipulation is the rapidly and easily visible individuality of the adhesive tape closure. Normal adhesive packaging tapes, therefore, are printed on the reverse of the backing with the individual company logo, so that a switch or overtaping with a different adhesive tape is readily perceptible. The possible print run of adhesive tapes identified in this way is a single roll at minimum, so that a high level of individuality of the delivery of the packages sealed using this adhesive tape may be achieved.
To date there has been no satisfactory success in combining the two security effects of message transfer and individual printing. Subsequent printing on the reverse of the backing of the existing adhesive tapes is impossible using the standard printing techniques, owing to the necessary silicone release on this surface. In the prior art embodiments of such an adhesive tape, therefore, the backing film is printed prior to the provision of the layers necessary for the message transfer effect. A consequence of this is that only large runs of adhesive tapes with an individual print can be produced economically.
Available embodiments of an adhesive security tape in the sense described are printed, as an alternative, with a serial number for the purpose of individualization. This method, however, requires a high level of information exchange within the logistics chain. In addition, it necessitates laborious checking of each delivery unit for agreement with the circle of numbers notified by the producer.
The technical implementation of an adhesive security tape of this kind is very costly in the material and in the production process. For instance, backing films used to date have been those comprising expensive materials such as polyesters and polystyrene. Films based on polystyrene, however, are brittle and do not withstand the impact and drop loads which occur during transport and warehousing of packages sealed with adhesive packaging tapes.
Thermoplastic films based on polyethylene terephthalate (PET) or polyvinyl chloride (PVC) are used by various manufacturers to produce adhesive tapes. Films based on PET, in particular, are notable for a high elongation at break and heat resistance of from 130xc2x0 C. to 175xc2x0 C., in addition to resistance to dilute alkalis and acids.
Furthermore, films based on polyesters possess very high abrasion resistance and puncture resistance, but are less widespread in the adhesive packaging tapes sector owing to their relatively high price relative to polyolefin-based films.
Films based on PVC are used, in combination with solventborne adhesive compositions based on natural rubber, by various manufacturers as adhesive packaging tapes. In this case, unplasticized PVC films are used owing to their better heat resistance, which extends up to max. 105xc2x0 C.
The PVC in this case may be obtained by emulsion, suspension or bulk polymerization. Copolymers based on PVC/vinyl acetate are also known.
Furthermore, the films based on unplasticized PVC are notable for moderate puncture strength and partial resistance to gasoline, oil and alcohols. In general, in the absence of corona treatment, the anchoring of primers to PVC films is better than in the case of other thermoplastic films, such as polyolefin-based films, for example. This is a result of the high surface energy even without pretreatment [approximately 39 dyn/cm] and on the rough surface texture of PVC films. For adequate anchoring of adhesive compositions based on natural rubber, a primer-coated PVC film is advisable.
In the production of adhesive tapes with adhesive compositions based on natural rubber, both water-based and solventborne primers are used as adhesion promoters between adhesive composition and backing film. These adhesion promoters which are used possess in part a crosslinking effect on the adhesive composition based on natural rubber which is applied from solution.
It is an object of the present invention to provide an adhesive security tape which on removal from the surface of a packaging leaves an especially colored layer, within which there is inverse writing. At the same time, the identical writing is visible in positive form on the peeled-off film backing.
This object is achieved by means of an adhesive tape as specified in the main claim. The subclaims relate to advantageous developments of the subject matter of the invention. The invention additionally relates to proposed uses of the adhesive tape of the invention.
The invention accordingly provides a multilayer adhesive tape having a backing based on an oriented thermoplastic film, whose upper face is coated, if desired, with a release coating and to whose lower face the following layers have been applied:
a primer layer which is not applied over the full area and which anchors very well to the thermoplastic film,
a release layer which is applied over the full area of the applied primer layer, and the free thermoplastic film surface, the release layer anchoring better to the primer layer than to the thermoplastic film,
a pressure-sensitive adhesive composition which is applied over the full area of the release layer and which is anchored better on the release layer than the release layer on the thermoplastic film.
When the adhesive tape of the invention is removed from the surface of a package, it leaves inverse writing behind on the package by virtue of the fact that the especially colored first release layer separates from the thermoplastic film, while areas which were firmly anchored to the backing surface by means of primer are detached from the surface of the package together with the backing.
The region of the first release layer which was strongly anchored to the film surface by the primer layer remains on the backing surface.
An appropriate gradation of the release forces of the multilayer construction produces reliable separation within the adhesive tape and the splitting of the package surface, especially in the case of cardboard packaging, is avoided by the adhesive composition. The writing may be reliably transferred, in particular, to all standard commercial paper grades for cardboard boxes.
The message is imperceptible prior to the detachment of the adhesive tape from the surface. Following detachment, the message remains perceptible even if the adhesive tape is replaced exactly in the original bond site, so that the evidence of a manipulation of quantitative or qualitative nature remains perceptible on the packaged product.
The adhesive security tape ensures that the message is transferred reliably to
a) solid surfaces which are difficult to split,
b) surfaces with little splitting resistance, and
c) surfaces having antiadhesive properties, as are customary in the case of standard commercial paper grades from waste paper.
Furthermore, the adhesive security tape of the invention is designed in such a way that economic individual printing for a minimum of one roll is possible.
In a first advantageous embodiment of the adhesive tape, the thermoplastic film comprises oriented HDPE, PVC, PET, monoaxially oriented polypropylene, or biaxially oriented polypropylene, and in particular is transparent.
Polyester-based films, polyethylene terephthalate for example, may likewise be used to produce the adhesive tapes of the invention.
The thicknesses of the films based on PET are between 20 and 100 xcexcm, in particular between 30 and 50 xcexcm.
Films based on oriented polyolefins belong to the prior art. Monoaxially and biaxially oriented films based on polyolefins are used in large amounts for adhesive packaging tapes, strapping tapes, and other adhesive tapes. Also known are films based on oriented polyethylene or oriented copolymers containing ethylene and/or propylene units. All of these listed films may be used as the backing film in accordance with the invention.
Monoaxially oriented polypropylene is notable for its very high tensile strength and low stretching in the longitudinal direction and is used, for example, to produce strapping tapes. For producing the adhesive tapes of the invention for the bundling and palletization of cardboard boxes and other goods, preference is given to monoaxially oriented films based on polypropylene.
The thicknesses of the monoaxially oriented films based on polypropylene are preferably between 25 and 200 xcexcm, in particular between 40 and 130 xcexcm.
Films of monoaxially oriented polypropylene are particularly suitable for producing the adhesive tapes of the invention. Monoaxially oriented films are predominantly single-layer films, although in principle multilayer monoaxially oriented films may also be produced. The known films are predominantly one-, two- and three-layer films, although the number of layers chosen may also be greater.
For the production of the adhesive tapes of the invention for secure carton sealing, preference is given to biaxially oriented films based on polypropylene with a draw ratio in the longitudinal (machine) direction of between 1:4 and 1:9, preferably between 1:4.8 and 1:6, and a draw ratio in the transverse (cross) direction of between 1:4 and 1:9, preferably between 1:4.8 and 1:8.5.
The moduli of elasticity achieved in the longitudinal direction, measured at 10% elongation in accordance with ASTM D882, are usually between 1000 and 4000 N/mm2, preferably between 1500 and 3000 N/mm2.
The thicknesses of the biaxially oriented films based on polypropylene are in particular between 15 and 100 xcexcm, preferably between 20 and 50 xcexcm.
Biaxially oriented films based on polypropylene may be produced by means of blown film extrusion or by means of customary flat film units. Biaxially oriented films are produced both with one layer and with a plurality of layers. In the case of the multilayer films, the thickness and composition of the different layers may also be the same, although different thicknesses and compositions are known.
Particularly preferred for the adhesive tapes of the invention are single-layer, biaxially or monoaxially oriented films and multilayer, biaxial or monoaxial films based on polypropylene which have a sufficiently firm bond between the layers, since delamination of the layers in the course of the application is disadvantageous.
Films based on unplasticized PVC are likewise known and are used by various adhesive tape manufacturers to produce adhesive packaging tapes. Plasticized PVC films are likewise suitable for producing adhesive packaging tapes, although their inadequate heat resistance of max. 60xc2x0 C. must be taken into account.
For the adhesive tapes of the invention for secure carton sealing it is preferred to use films based on unplasticized PVC. The thicknesses of the films are preferably between 30 and 100 xcexcm, in particular between 35 and 50 xcexcm. The adhesive tapes of the invention are used as packaging tapes for carton sealing.
Release coatings as may be applied to the upper film surface, i.e., to the film surface opposite the adhesive composition, are in widespread use in the adhesive tape industry. An overview of release coating systems based on silicone and their use in the pressure-sensitive adhesives field may be found in xe2x80x9cSilicone Release Coatingxe2x80x9d (D. Jones, Y. A. Peters in Handbook of Pressure Sensitive Adhesive Technology, Third Edition, edited by Donatas Satas, Van Reinhold New York, pp. 652-683).
The use of rear-face release coatings results in easy, uniform unwinding of the adhesive tapes. Used above all in this context are release coatings based on solventborne carbamate systems and on silicone-based systems.
Release coatings based on silicone systems are suitable as release coatings of the adhesive tapes of the invention. Preferred release coatings are UV-curing silicone systems on a 100% basis, although solventborne silicone systems may likewise be used as a release coating for the adhesive tapes of the invention.
The application rate of the release coating on the backing material is preferably from 0.1 to 3 g/m2. In a further, particularly preferred embodiment, the application rate of the release coating layer is set at from 0.2 to 1.5 g/m2.
In order to achieve very good anchoring of the first release layer of the adhesive tape of the invention on specific regions of the film, an adhesion promoter or primer is used as the first layer. In this case, the film surface opposite the release coating is used.
Both solvent- and water-based adhesion promoters are used as adhesion promoters between film and adhesive composition or further layers for the production of adhesive tapes. Layers with high affinity to backings or adhesion to backings based on thermoplastic films are used to produce adhesive tapes. The high affinity to surfaces of backings based on thermoplastic films governs the quality of such components for use as appropriate adhesion promoters. A variety of primer systems is known. Adhesion promoters suitable for use as the first release layer of the adhesive tapes of the invention are, for example, aqueous adhesion promoters based on polyurethanes, polypropylenes with low degrees of chlorination, styrene-butadiene copolymers, and acrylates. Solventborne adhesion promoters are likewise suitable for the anchoring of subsequent layers in the adhesive tapes of the invention. Within the adhesives tape industry, these adhesion promoters are used as primers.
The application rate of the primer layer to the backing material is preferably from 0.5 to 12 g/m2. In a further preferred embodiment, the layer application rate of the first adhesion promoter layer is set at from 1.5 to 6 g/m2.
In one preferred embodiment of the adhesive tape, the adhesion of the primer layer to the polyolefin-based thermoplastic film is improved by means of corona treatment or, especially, by means of flame pretreatment, since the surfaces of films based on oriented polyolefins in particular may be treated by means of these widely known processes, such as corona treatment or flame treatment. Preference is given to surface treatments by means of flame pretreatment. An overview of the techniques for surface treatment is contained, for example, in the article xe2x80x9cSurface pretreatment of plastics for adhesive bondingxe2x80x9d (A. Kruse; G. Krxc3xcger, A. Baalmann and O. D. Hennemann; J. Adhesion Sci. Technol., Vol. 9, No 12, pp 1611-1621 (1995)).
The primer layer is not applied over the whole area of the thermoplastic film-based backing surface opposite the release coating. In order to achieve a visual security effect in the application of the adhesive tapes of the invention, the primer layer is applied in a specific, incomplete manner to the film surface. The application of the primer layer is designed in such a way that only specific regions of the film surface are coated which form, for example, the word xe2x80x9cOPENEDxe2x80x9d or xe2x80x9cVOIDxe2x80x9d. This mode of application of the primer layer is made possible by means of appropriate printing processes.
The incompletely applied primer layer on the film surface preferably constitutes characteristic signs and/or symbols.
A release layer is applied over the full area of the primer layer and/or to the free surface regions of the film used, for the production of the adhesive tapes of the invention. Examples of suitable release layers for the adhesive tapes of the invention are resin dispersions, acrylate dispersions or, in general, polymer dispersions, and/or solventborne polymer solutions, which exhibit a brittleness and a low adhesion to the backing which is sufficient after drying.
Particularly preferred release layers for the adhesive tapes of the invention are appropriate printing inks for backings based on thermoplastic films. For this layer, use is made, for example, of water-based, solvent-based or UV-curing flexographic inks. Ink layers of this kind are strongly colored and suitable for application to the first layer and/or the film by means of a wide variety of technologies. Through an appropriate choice of the printing inks it is possible to formulate the layer system of the adhesive tapes of the invention in such a way that the layer is very well anchored to the primer layer but exhibits poor adhesion, or a poor bond, with the film regions that have not been coated with the primer layer.
The application rate of the release layer to the backing material and/or the primer layer is preferably from 2 to 18 g/m2. In a further preferred embodiment, the layer application rate of the first layer is from 5 to 11 g/m2.
As adhesive compositions it is possible to use essentially all known adhesive compositions with high bond strength on the adhesion substrate which is to be packaged.
The adhesive composition of the adhesive tape of the invention may comprise an adhesive composition based on solventborne natural-rubber and acrylate adhesive compositions. Preference is given to adhesive compositions based on acrylate dispersions. These adhesive composition technologies are used in the adhesive tape industry.
The application rate of the adhesive composition to the backing material is preferably from 15 to 60 g/m2. In another preferred embodiment, the layer application rate is set at from 20 to 30 g/m2.
The adhesive tapes of the invention are produced in a multistage process. In a first step, one of the surfaces of the thermoplastic film-based backing is coated with the silicone-based release coating. This single-sided coating of the backing is realized using corresponding coating units.
The film surface opposite the release coating is pretreated by corona or flaming if desired.
The primer layer with high affinity for the surface of the thermoplastic film used is applied by means of flexographic printing to the film surface opposite the release coating.
Printing by means of the flexographic process is used, inter alia, in the adhesive tape industry for the individualization of adhesive tapes.
It is carried out using printing forms whose raised parts transfer the first layer. The printing forms are flexible plates made of rubber or photopolymers. Flexographic printing components are highly mobile, both solvent-based and water-based. They dry rapidly, especially in the area of the radiation-curing systems. For the production of the adhesive tapes of the invention, it is preferred to use aqueous flexographic printing components. The selection of the screen roller is guided by the printing material used and by the associated flexographic printing component.
For the production of the adhesive tapes of the invention, the application of the release layer to the primer layer and/or to the open areas on the film surface is achieved by the use of customary methods. Mention may be made in this context of the application of aqueous systems by means of knife coaters or of solventborne systems by means of bar coaters, screen rollers or printing processes.
For the subsequent application of the adhesive composition to the layer, both the bar coater and the knife coating technology are suitable.
Also possible is the application of adhesive composition based on styrene-isoprene-styrene as hot-melt pressure-sensitive adhesive to the release layer by means of known technology, in the context of the production of the adhesive tapes of the invention.